Many methods are available to estimate cylinder air charge using a manifold pressure sensor. Typically, engine maps are provided that provide a cylinder air charge as a function of measured manifold pressure, manifold temperature, and engine speed. In engines that also utilize exhaust gas recirculation, an improved cylinder air charge estimate is obtained by providing adjustments based on the amount of exhaust gas recirculation.
One particular method is described in U.S. Pat. No. 5,205,260. In this method, an EGR flow is estimated based on differential pressure across a flow control valve and based on a cross-sectional area of the valve. Then, this flow is used in a manifold filling model to estimate a partial pressure of EGR in the intake manifold. Then, based on this partial pressure of EGR and measured manifold pressure, a cylinder air charge value is computed.
The inventors herein have recognized a disadvantage with the above system. In particular, estimating EGR flow in this manner leads to estimation inaccuracies. Since inaccuracies in EGR flow directly affect estimated cylinder air charge, this leads to inaccuracies in calculated fuel injection amount and therefore may degrade air-fuel ratio control.